Subsonic islands within a high-mass star-forming IRDC

Abstract

High-mass star forming regions are typically thought to be dominated by supersonic motions. We present combined Very Large Array and Green Bank Telescope (VLA+GBT) observations of NH${}_3$ (1,1) and (2,2) in the infrared dark cloud (IRDC) G035.39-00.33, tracing cold and dense gas down to scales of 0.07 pc. We find that, in contrast to previous similar studies of IRDCs, more than a third of the fitted ammonia spectra show subsonic non-thermal motions (mean line width of 0.71 $\mathrm{km}{\mathrm{s}}^{-1}$), and the sonic Mach number distribution peaks around $\mathcal{M}=1$. As possible observational and instrumental biases would only broaden the line profiles, our results provide strong upper limits to the actual value of $\mathcal{M}$, further strengthening our findings of narrow line widths. This finding calls for a reevaluation of the role of turbulent dissipation and subsonic regions in massive-star and cluster formation. Based on our findings in G035.39, we further speculate that the coarser spectral resolution used in the previous VLA NH${}_3$ studies may have inhibited the detection of subsonic turbulence in IRDCs. The reduced turbulent support suggests that dynamically important magnetic fields of the 1 mG order would be required to support against possible gravitational collapse. Our results offer valuable input into the theories and simulations that aim to recreate the initial conditions of high-mass star and cluster formation.

arXiv comment5 pages, 3 figures; accepted for publication in A&A Letters; arxiv revision matches accepted version

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